1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display operated with small power consumption is characterized by small thickness and light weight, and there is a growing demand for the display mainly for a computer application. Since a liquid crystal, which is a component member of the liquid crystal display, does not emit light by itself, an illuminating means is required for observing an image unlike a light-emitting element such as a CRT. To answer a recent request for reduction in thickness, a spread illuminating apparatus of thin plate side-light type (light-conductive plate type) is extensively used as an illuminating means to illuminate the liquid crystal display.
The applicant of the present invention discloses one embodiment of the spread illuminating apparatus of such a side-light type in the Japanese Patent Application Laid-open No. 11-242222. The spread illuminating apparatus 1 is disposed so as to cover an observation surface F of a reflection type liquid crystal element 51, and a light source lamp 52 is disposed along an end surface 3a of a substantially rectangular transparent substrate 2 with a specified distance therefrom, as illustrated in FIGS. 3 and 4.
A light reflection pattern 12 is formed on an upper surface 11a of the transparent substrate 2 in such a manner that brightness is almost uniform everywhere on the transparent substrate 2 irrespective of the distance from the light source lamp 52. The light reflection pattern 12 comprises a plurality of grooves 13 substantially triangular in section and a plurality of flat portions 14 adjacent to the grooves 13.
The grooves 13 each comprise one inclined surface (hereinafter referred to as xe2x80x9cfirst inclined surfacexe2x80x9d) 30 positioned closer to the light source lamp 52, i.e. closer to one end surface 3a and another inclined surface (hereinafter referred to as xe2x80x9csecond inclined surfacexe2x80x9d) 40 positioned closer to another end surface 3b opposite to the end surface 3a. 
The flat portions 14 are parallel to a lower surface 11b of the transparent substrate 2, and therefore the height of an upper edge 31 of the first inclined surface 30 is equal to the height of an upper edge 41 of the second inclined surface 40 with respect to the lower surface 11b. 
In the above-described conventional art, part of light rays 60, which are emitted from the light source lamp 52 and strike the first inclined surface 30, may exit out the transparent substrate 2 passing the first inclined surface 30 as leakage light M failing to contribute to illumination. The ratio of the amount of leakage light to the amount of light entering the transparent substrate 2 is 10 to 20% leading to a considerable decrease of illumination efficiency. Further, the leakage light as described above causes a deterioration in contrast on a display surface.
The present invention has been made in the light of the above problems, and it is an object of the present invention to provide a spread illuminating apparatus suppressing the leakage of light.
In order to solve the above problems, according to a first aspect of the present invention, there is provided a spread illuminating apparatus in which a transparent substrate made of a light-transmissible material is disposed so as to cover a surface of a member to be illuminated, a bar-like light source is disposed close to one end surface of the transparent substrate and a light reflection pattern comprising a plurality of grooves having a first inclined surface and a second inclined surface, shaped substantially triangular in section, extending parallel to the optical axis of the light source and ranging from the one end surface of the transparent substrate to another end surface opposite to the one end surface and a plurality of flat portions formed between the plurality of grooves is provided on an upper surface of the transparent substrates, wherein an upper edge of the second inclined surface formed closer to the another end surface of the transparent substrate is set to be positioned higher than an upper edge of the first inclined surface formed closer to the one end surface with respect to a predefined datum level, and therefore the flat portions are inclined in such a manner as to descend from the one end surface toward the another end surface.
According to a second aspect of the present invention, in the spread illuminating apparatus of the first aspect, the height defined by the distance between the upper edge of the second inclined surface and the bottom level of the grooves is twice or less as large as the height defined by the distance between the upper edge of the first inclined surface and the bottom level of the grooves.
According to a third aspect of the present invention, in the spread illuminating apparatus of the first or second aspect, the inclination angle of the flat portions with respect to the datum level is 5xc2x0 or less.
According to a fourth aspect of the present invention, in the spread illuminating apparatus of any one of the first to third aspects, the inclination angle of the first inclined surface with respect to the datum level is 35xc2x0 to 55xc2x0 and the inclination angle of the second inclined surface with respect to the datum level is 70xc2x0 or more.
According to a fifth aspect of the present invention, in the spread illuminating apparatus of any one of the first to fourth aspects, the transparent substrate is formed uniform in thickness as a whole from the one end surface to the another end surface, where the datum level is a virtual plane defined by connecting the upper edges of the first inclined surfaces.
According to a sixth aspect of the present invention, in the spread illuminating apparatus of any one of the first to fourth aspects, the transparent substrate is of a wedge shape as a whole in which the thickness thereof is in inverse proportion to the distance from the one end surface, where the datum level is a virtual plane defined by connecting the upper edges of the first inclined surfaces.